How about F? This one is easy too. The interval between F and C is also a perfect fifth. But this time we’re going down a fifth, not up. So to get down to F, we’re going to divide C’s frequency by 3/2, which gives us 2/3 Hz. We can then bring it up an octave by doubling its frequency, giving us an F at 4/3 Hz.
There’s an old studio saying: “Crap in, crap out.” No amount of mixing is going to save a bad vocal recording, so it’s important that you get it right at the source. Unfortunately, you probably don’t have much say over the recording process if you’re already at the point of mixing. But if you can get your hands on the raw tracks, you’ll be able to cherry-pick your favorite words or lines from each take and comp them together to create the ultimate Frankenstein-style performance.